<p>This study systematically investigated the interfacial reactions between Co and Sn-3.5Ag-Ge solders, with Ge concentrations ranging from 0.03 wt.% to 2 wt.%, at 250°C under liquid-state conditions. Additionally, the phase equilibria of the Sn-Ge-Co system were partially determined in the compositional region below 50 at.% Co. The interfacial microstructural evolution exhibited a pronounced dependence on Ge concentration. At a trace addition of 0.03 wt.% Ge, CoSn₃ formed exclusively at the interface. Increasing the Ge content to 0.05 wt.% triggered a transitional reaction regime, in which CoSn₃ coexisted with a newly identified ternary intermetallic compound (IMC), designated as the X phase. With a further increase to 0.1 wt.% Ge, the nucleation of CoSn₃ was effectively suppressed, resulting in a stable interface dominated by the X phase. At higher Ge concentrations (up to 0.3 wt.%), the interfacial reaction evolved toward the formation of plate-like Co₅Ge₇, together with the X-phase layer, with Co₅Ge₇ progressively coarsening into a granular morphology. Notably, upon reaching 1 wt.% Ge, Co₅Ge₇ became the predominant and stable interfacial product. However, the presence of microcracks with Co₅Ge₇ suggests its intrinsically brittle nature, which may compromise solder joint reliability. These findings, supported by the constructed Sn-Ge-Co phase equilibria, provide a comprehensive understanding of how Ge addition governs IMC phase stability and interfacial evolution in Sn-Ag-Ge/Co systems.</p>